1. Field of the Invention
The present invention relates to an image detection device having an image detector in which a large number of pixels having TFT switching elements are arrayed two-dimensionally, and to a method of driving an image detector.
2. Description of the Related Art
FPDs (flat panel detectors), in which an X-ray sensitive layer is disposed on a TFT active matrix array and which can directly convert X-ray information into digital data, have been put into practical use in recent years. As compared with a conventional imaging plate, an FPD has the advantages that an image can be confirmed promptly and moving images as well can be confirmed, and the spread of FPDs has advanced rapidly.
First, the structure of a conventional radiographic image detector will be described by using FIG. 15. FIG. 15 is a schematic equivalent circuit of 3×3 pixels. In FIG. 15, reference numeral 211 is an image sensor portion, 212 is a TFT switch, 213 is a charge storage capacitor, 214 are scan lines, 215 are data lines, and 216 are signal detectors.
Radiation such as X-rays or the like is incident on the image sensor portions 211, electron-hole pairs are generated, and the charges are accumulated in the storage capacitors 213 of the respective pixels. Thereafter, the TFT switches 212 are successively turned on by the scan lines 214, and the accumulated charges that are accumulated in the charge storage capacitors 213 are transferred to the data lines 215 that are connected to ones of the source/drain electrodes of the TFT switches 212, and are read by the signal detectors 216.
Such a radiographic image detector that is called an FPD has the feature that, because image signals can be detected directly, an accurate radiographic image is detected. However, there are cases in which, for any of various reasons, various types of noise are imparted to the image signal that is to be detected originally.
For example, leak current of a TFT switch is one type of noise. It is preferable that absolutely no leak current flow at the time of the off operation of the TFT switch for selecting the detection pixel. However, in light of the device characteristics, a given amount of leak current flows, and that amount is added to the image signal. In order to overcome this problem, Japanese Patent Application Laid-Open (JP-A) No. 2003-319264 for example proposes a method of reading out leak current at the time when a TFT switch is off, and correcting the image signal by using the leak current value.
However, the quality of the image does not necessarily improve with the method described in JP-A No. 2003-319264. The reason for this is because, in the method of JP-A No. 2003-319264, the processing of subtracting the leak current component from the image signal is used in correcting the leak current amount. Because the leak current component is random noise, even if the leak current component is subtracted from the image signal, the result is the addition value of the image signal and the leak current component, and the amount of noise increases. Further, there is the problem that there is the possibility that this will resultantly lead to a deterioration in image quality.
Further, the processing time required for the aforementioned leak correction also is an important problem. In medical diagnoses, because the photographing conditions must be optimized in accordance with the patient, the frequency of retaking images is high, and an image must be confirmed at a stage as soon as possible immediately after photographing. Further, X-ray detectors for moving images of course cannot take any more time than the frame rate. Moreover, the cost of peripheral circuits such as a memory and the like needed for correction processing is high, and when aiming to make the processing time faster, an even more expensive circuit system is needed.